Sensors are transducers or converters that measure a physical quantity and convert it into a signal which can be read. Typically, that reading is by an electronic instrument which converts the signal to a measurement based upon the sensitivity of the sensor, its calibration data, and other corrections. Included within the many types of sensors are those relating to sound, acoustics, vibration, chemicals, humidity, pressure, fluid flow, position, displacement, force, level, temperature, proximity, and acceleration. For each type of sensor, different sensing mechanisms exist which may for example be targeted to different dynamic ranges, speed, accuracy, etc. Amongst these capacitive sensing constitutes a very important means of monitoring and accordingly, capacitive sensors have major applications in the consumer, industrial, automotive and medical fields.
However, in essentially all applications, the important considerations for selecting a capacitive sensor include, but are not limited to, accuracy, repeatability, long-term stability, ease of calibration, resistance to chemical and physical contaminants, size, packaging, integration options with other sensors and/or electronics, and cost effectiveness. It is usually desirable to miniaturize and integrate such capacitive sensing systems in order to meet the requirements of existing markets and penetrate new markets and reduce fabrication costs through batch processing. Sustainable protection from oxidation and corrosion and ruggedness are also especially critical for operation in harsh environments. In many instances, the integration of capacitive based sensors directly with their associated electronics is important in attaining packaging dimensions and costs that are compatible with very low-cost high volume markets, such as consumer electronics for example. Additionally, the ability to integrate multiple capacitive sensors within a single compact low cost system is beneficial as it reduces system footprint and overall assembly costs.
Microelectromechanical systems (MEMS) are small integrated devices or systems that combine electrical and mechanical components. The components can range in size from the sub-micrometer level to the millimeter level, and there can be any number, from one, to few, to potentially thousands or millions, in a particular system. Historically, MEMS devices have leveraged and extended the fabrication techniques developed for the silicon integrated circuit industry, namely lithography, doping, deposition, etching, etc. to add mechanical elements such as beams, gears, diaphragms, and springs to silicon circuits either as discrete devices or in combination with silicon electronics. Examples of MEMS device applications today include inkjet-printer cartridges, accelerometers, miniature robots, micro-engines, locks, inertial sensors, micro-drives, micro-mirrors, micro actuators, optical scanners, fluid pumps, transducers, chemical sensors, pressure sensors, and flow sensors. These systems can sense, control, and activate mechanical processes on the micro scale, and function individually or in arrays to generate effects on the macro scale. The micro fabrication technology enables fabrication of large arrays of devices, which individually perform simple tasks, or in combination can accomplish complicated functions.
Silicon CMOS electronics has become the predominant technology in analog and digital integrated circuits. This is essentially because of the unparalleled benefits available from CMOS in the areas of circuit size, operating speed, energy efficiency and manufacturing costs which continue to improve from the geometric downsizing that comes with every new generation of semiconductor manufacturing processes. In respect of MEMS systems, CMOS is particularly suited as digital and analog circuits can be designed in CMOS technologies with very low power consumption. This is due, on the digital side, to the fact that CMOS digital gates dissipate power predominantly during operation and have very low static power consumption. This power consumption arising from the charging and discharging of various load capacitances within the CMOS gates, mostly gate and wire capacitance, but also transistor drain and transistor source capacitances, whenever they are switched. On the analog side, CMOS processes also offers power savings by offering viable operation with sub-1V power supplies and with μA-scale bias currents.
Amongst the many environmental parameters electrical and magnetic field (EMF) exposure has been the subject of substantial research and analysis. Health-related research around EMF has focused primarily on magnetic field exposure. However, magnetic fields are not easily sensed and accordingly today micromachined Lorentz force based magnetometers are receiving considerable attention in the sensing community, as they can be fabricated without requiring any custom magnetic materials (e.g., integrated permanent magnets in the device of Ettelt et al., “A Novel Microfabricated High Precision Vector Magnetometer” (Proc. IEEE Conf. on Sensors, pp. 2010-2013). The ability to remove magnetic materials allows for the co-fabrication of magnetometers alongside other MEMS sensors on the same chip, for augmented functionality with minimum impact on form factor. Such integration is highly attractive for consumer electronics applications, where MEMS sensors are playing an increasing role each day. In such cost-sensitive applications, exotic magnetic materials often do not justify the added costs and fabrication complexity, and limit the compatibility of magnetometers with other MEMS sensing structures and integrated circuits (ICs). Within the prior art an out-of-plane Lorentz force magnetometer and a ferromagnetic in-plane nickel magnetometer have been reported requiring high temperature fabrication steps, e.g., 800° C. In other research Lorentz force-based resonant sensors for in-plane or out-of-plane magnetic fields have been reported using silicon-on-insulator (SOI) technology. While two similar orthogonal structures could be used for 3D sensing, these devices are not suitable for post-CMOS monolithic integration. Chang et al. in “Development of Multi-Axis CMOS-MEMS Resonant Magnetic Sensor Using Lorentz and Electromagnetic Forces” (Proc. IEEE Conf. on MEMS, pp. 193-196) co-fabricated CMOS electronics and MEMS magnetometer exploiting the commercial TSMC 0.35 μm technology but this restricts the materials and inherits the constraints inherent to that semiconductor process node.
Alternatively, sensors that are amenable to above-IC integration present lower parasitics to the associated readout circuitry, resulting in an improved overall sensitivity. Furthermore, the area sharing made possible by overlaying the sensors with the electronics allows for smaller overall chip size, compared to the side-by-side co-fabrication approach. Accordingly, the inventors have established a combined Lorentz force based magnetometer and accelerometer MEMS sensor exploiting a low temperature, above-IC-compatible fabrication process. The proposed sensor exploits switching an electrical current between two perpendicular directions on the device structure to achieve a 2D in-plane magnetic field measurement. Concurrently, the device serves as a 1D accelerometer for out-of-plane acceleration, by switching the current off and by monitoring the structure's capacitive change in response to acceleration. The design can thus separate magnetic and inertial force measurements, utilizing a single compact device. The combined Lorentz force based magnetometer and accelerometer MEMS sensor supports static operation at atmospheric pressure removing the requirements for complex vacuum packaging. However, the device can also be packaged under vacuum allowing it to operate at resonance for enhanced sensitivity. The device is fabricated using a silicon carbide (SiC) surface micromachining technology established by the inventors which is fully adapted for above-IC integration on standard CMOS substrates.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.